你如何构造一个递归函数？

Question

我正在尝试创建一个递归函数来检查成绩是否可以转换为浮点数，然后检查以确保成绩在 0 到 100 之间。我关于为什么会这样的逻辑是：

1. 检查成绩是否可以转换为浮点数。
2. 如果不能转换成浮点数，用户会收到错误信息，必须输入新的成绩，gradeChecker 会再次检查。
3. 如果可以转换为浮点数，gradeChecker 会检查它是否在 0 到 100 之间
4. 如果成绩介于 0 和 100 之间，则该成绩将附加到“作业”字典中。
5. 如果分数不在 0 到 100 之间，用户会给出一个新分数，并将其传回函数，因此从第 1 步开始重复。

你能告诉我这段代码怎么不符合那个逻辑吗？

def gradeChecker(grade):
    while True:
        try:
            grade = float(grade)        
            while (grade < 0) or (grade > 100):
                print("Sorry, your grade must be between 0 and 100.")
                grade = input("What grade did you receive on: %s? " % assignment)
                gradeChecker(grade)
        except:
            print("Sorry, that's an invalid input. Please only use numbers and decimal points!")
            grade = input("What grade did you receive on: %s? " % assignment)
        else:
            break
    Assignments[assignment].append(grade)

编辑：缩进已修复！

作为参考，这里是我用来将成绩传递到gradeChecker 的全部代码。

import collections #We import collections so we can use an ordered dictionary
Assignments = collections.OrderedDict() #We create an ordered dictionary called 'Assignments'
#We have to add each assignment to our dictionary individually because it's an ordered dictionary
#You can't convert a regular dictionary, which is unordered, to an ordered dictionary
#If you used a regular dictionary, when looping through the assignments they would be displayed at random
Assignments['Exam 1'] = [0.2] #We add the assignment and the weight to the dictionary
Assignments['Exam 2'] = [0.2] #We add the assignment and the weight to the dictionary
Assignments['Exam 3'] = [0.2] #We add the assignment and the weight to the dictionary
Assignments['Homework'] = [0.2] #We add the assignment and the weight to the dictionary
Assignments['LQR'] = [0.1] #We add the assignment and the weight to the dictionary
Assignments['Final'] = [0.1] #We add the assignment and the weight to the dictionary

#We have to define our grade checker before it is called so that it can be used to verify users inputs

def gradeChecker(grade): #Used to verify that a users grades are in-between 0 and 100
    while True:
        try:
            grade = float(grade)        
            while (grade < 0) or (grade > 100):
                print("Sorry, your grade must be between 0 and 100.")
                grade = input("What grade did you receive on: %s? " % assignment)
                gradeChecker(grade)
        except:
            print("Sorry, that's an invalid input. Please only use numbers and decimal points!")
            grade = input("What grade did you receive on: %s? " % assignment)
        else:
            break
    Assignments[assignment].append(grade) #We append the grade to the assignment to be used with it's weight for the final grade calculation

#INPUT

for assignment in Assignments.keys(): #We loop through every assignment in our  Assignments dictionary
    grade = input("What grade did you receive on: %s? " % assignment) #We ask the user to enter their grade
    gradeChecker(grade) #We check to see if their grade is valid by passing their input through the gradeChecker function

Answer 1

在这里同时使用递归和循环是多余的。此外，循环应该优先于 Python 等“普通”语言（即不是 Lisp）中的递归，除非您正在处理像树这样的固有递归数据结构。

鉴于你从来没有说过为什么你首先需要使用递归，我建议使用这样的东西：

def input_grade(assignment):
    while True:
        try:
            grade = input("What grade did you receive on: %s? " % assignment)
            grade = float(grade)
            if grade < 0 or grade > 1000:
                raise ValueError
            return grade
        except:
            print("Sorry, that's an invalid input. Please only use numbers and decimal points!")

for assignment in Assignments:
    Assignments[assignment].append(input_grade(assignment))

注意 1。我没有测试上面的代码，但它应该能让你大致了解如何构造这个函数。

注意 2。你应该避免在函数中修改Assignments，这是一个不需要的副作用。最好将其卸载到主代码中。

注 3。从技术上讲，我的代码中的 try..except 是基于异常的逻辑的示例，这不是一件好事（但是，这比在这里使用递归要好得多）。

Answer 2

def gradeChecker(grade):
    try:
        grade = float(grade)        
        while (grade < 0) or (grade > 100):
            print("Sorry, your grade must be between 0 and 100.")
            grade = input("What grade did you receive on: %s? " % assignment)
            gradeChecker(grade)
    except:
        print("Sorry, that's an invalid input. Please only use numbers and decimal points!")
        grade = input("What grade did you receive on: %s? " % assignment)
    else:
        Assignments[assignment].append(grade)

您的代码的问题是它是多余的； while 语句可以做你已经有递归调用做的事情。 imo，while 语句更好，因为递归通常会占用更多内存，但这只是一种意见。